1. Field of the Invention
The present invention relates to a technology for manipulating movement of particles in a medium.
2. Related Background Art
The sheath flow method is already well known for transporting fine particles dispersed in a medium such as liquid, in linear array, one by one. In this method, at the center of a high-speed sheath liquid flow, there is formed a very fine flow of liquid in which fine particles such as blood cells, viruses, microorganisms or carrier particles (such as latex particles or ceramic particles) are suspended, whereby the fine particles are separated one by one, transported in linear array. Such sheath flow method is applied to so-called flow cytometry which is used for counting the number of such fine particles by an optical or electrical method or for identifying the sort or property of such particles by statistical processing of a number of thus obtained data.
On the other hand, for classifying the fine particles by size or specific gravity thereof, there are known methods utilizing centrifugal force or sieving means such as meshes or gels. These methods are applied to classify copying toner or other industrial particles, or to separate microorganisms, cells or the like.
Apart from these technologies, it is already known that, when fine particles are irradiated with a light beam, which has an intensity gradient such as Gaussian laser beam, the particles are subjected to scattering and gradient forces of radiation pressure so that the fine particles are moved in the direction of irradiation by an axial force and are trapped in the light beam by a radial force. This is a technology called optical trapping, disclosed, for example, in Japanese Patent Laid-Open Application Nos. 2-91545 and 3-223645 and in U.S. Pat. No. 3,808,550. Also, U.S. Pat. No. 4,887,721 discloses a method of switching two crossing laser beams, thereby transporting the fine particles to a predetermined position by laser optical pressure.